How to reproduce:
1. Open the attached '1301931.zip' project
2. In the Project window select the 'Test' Script Graph Asset
3. In the Inspector select 'Edit Graph'
4. Make sure that the Script Graph window is undocked
5. Press the 'Full Screen' button (located in the upper right corner of the Script Graph window)

Expected result: The Script Graph window enters fullscreen mode or the 'Full Screen' button is disabled
Actual result: The Script Graph window does not enter fullscreen mode and the 'Full Screen' button is not disabled

Reproducible with: Visual Scripting 1.5.0, 1.5.1-pre.3 (2021.1.0b1)
Cannot test with: Visual Scripting 1.5.0, 1.5.1-pre.3 (2018.4.30f1, 2019.4.17f1, 2020.1.17f1, 2020.2.1f1) (the package is not supported)

Notes:
- The issue does not reproduce when the Script Graph window is docked
- The fullscreen mode can be enabled by pressing on the three dots located in the upper right corner of the Script Graph window and selecting 'Maximize' when the window is docked or pressing the maximize button (with the square) located in the upper right corner between the 'X' button and three dots when the Script Graph window is undocked